1. Field of the Invention
The invention relates to a system and a method for the detection of the presence of a signal and its synchronization, especially in a frequency-hopping system working in a disturbed environment.
The invention can be applied especially to frequency-hopping systems that work at very low signal-to-noise ratios (particularly those using artificial satellites as relays). For these systems, it relates to the acquisition function which consists, for a participant who is entering the network and is therefore non-synchronized, in detecting the presence of known signals sent at known times and frequencies.
2. Description of the Prior Art
Present-day protected telecommunications systems implement a spread-spectrum method that uses a band for transmission that is far broader than the band strictly needed to transmit payload data.
There are two main principles in existence:                Spread-spectrum communications through the broadening of the band generally obtained by over-modulating the original signal by means of a pseudo-random sequence that is synchronous with the data and has a bit rate close to the total occupied bandwidth,        Spread-spectrum communications by frequency hopping (FH) in which the signal is sent by “stages>> or “levels” of a fixed duration, and in which the frequency is changed pseudo-randomly throughout the occupied band.        
In the present description, the term “pseudo-random” means “random for an intruder” and “perfectly known” to a participant in the system who has an ad hoc random generation device, as well as a “key” and a “time” common to all the participants.
FIG. 1 shows the principle of a frequency-hopping system in which the occupied band is limited by the two borderline frequencies F1 and F2 that have to work at very low signal-to-noise ratios (particularly those used by artificial satellites working as relays). For these systems, the occupied band is limited to the acquisition function which consists, for a participant entering the network—hence a participant who is not synchronized—in detecting the presence of known signals sent at known instants and frequencies.
Each stage has N elements which may or may not be carriers of information called symbols that are preceded (or followed, this is immaterial) by a guard time designed to minimize interference between approximately synchronized users and leave the receiver the time to “get stabilized” at each new frequency (i.e. the time corresponding to gain control, the frequency synthesizer positioning time, the smear of the filters, etc.).
In order to be able to carry out the initial synchronization, as stated further above, known stages (i.e. stages for which the N symbols are known wholly or in part) are sent according to a known law, while the majority of the other stages comprise (almost) only unknown symbols (payload information), as shown in FIG. 2.
It is assumed here (in order to simplify the description) that the time is divided into basic patterns of constant duration, each pattern comprising only a reference stage placed at a position known in advance.
FIG. 3 shows the action of the receiver during the search phase, i.e. when it does not yet know the “reference time” of the system to which it seeks to gain access.
Two situations are shown in this figure, corresponding respectively to the following two cases:
1) The receiver already has an excellent estimate of the time of the system (as seen in the shaded strips at the top): all it does therefore is to keep watch, for a duration equal to a basic pattern, on the frequencies at which the reference stages or levels are located. Thus, if the conditions are suitable, it will sooner or later detect both the presence and the precise position of said stages, from which it will deduce the time resetting that it must perform.
If it has detected nothing at the end of a certain period of time, it will put its clock forward or back by a quantity deduced from the duration of the basic pattern, and will make another attempt at detection.
2) The receiver has only a fairly imprecise estimation of the time of the system (shaded strips at the bottom): it will then keep watch for a duration that is a multiple of the duration of the basic pattern, the multiplier factor being especially great as its temporal uncertainty is greater. The frequency to be watched each time will logically be that of the reference stage which, in principle, should be in the basic pattern located in the middle of the watch duration. In all other respects, the same procedure is used as in the case 1.
When the signal-to-noise ratio is sufficient or when the probability of scrambling is not excessively high, the receiver uses a presence detection system as described in FIG. 4.
Depending on the local time of the receiver, a random generator delivers the number of the frequency to be watched as well as a binary string which is used to reconstitute the known part of the signal expected on the reference stage to be detected, the unknown part being set at zero.
After any unspecified form of level detection depending on the characteristics of the system and/or on the structure chosen for the receiver, the received signal is compared with the expected signal in a device called a “correlator” (equivalent to a filter “adapted” to the reference signals) whose output is the maximum when:                the received signal truly comprises the right reference symbols at the expected positions,        the received signal is perfectly “secured” in the correlator, i.e. each received reference signal “faces” the expected reference signal.        
Indeed, if S is the received signal and R is the reference, the output of the correlated at the instant t is given by:
      Corr    ⁡          (      t      )        =            ∑      n                            ⁢                  ⁢                  S                  t          +          nT                    ⁢              R        n        *            
where T is the interval between two reference symbols.
If we overlook the samples which are not references, the receiver signal can take the form:Sto+kT=GRk+Bk
where G is a complex gain and B is complex noise.
The output of the correlator at the instant to is then given by:
      Corr    ⁡          (              t        0            )        =            ∑      n                            ⁢                  (                              GR            n            *                    +                      B            n                          )            ⁢              R        n        *            
Or again:
      Corr    ⁡          (              t        0            )        =            G      ⁢                        ∑          n                                                ⁢                                                        R              n                                            2                      +                  ∑        n                                      ⁢                        B          k                ⁢                  R          n          *                    
Namely G times the sum of the squares of the moduli of the reference symbols with the addition of a random noise.
It can be shown that, statistically, Corr(to) is the maximum of the output of the correlator.
Accessorily, it can also be shown that if the frequency shift of the receiver signal relative to the nominal frequency is slightly too high, the level of correlation decreases rapidly and makes the detection impossible.
A simple solution then consists in considering not the directly received signal, but the signal after a differential demodulation having the form:Std=StSt−T*the correlation being done by replacing the original references with “differential” references, namely:Rnd=RnRn−1*
Thus, if the frequency shift of the signal is df, it can easily be shown that the output of the differential demodulator becomes:S′td=StSt−T*=StSt−T*=e−j2πdfTe−j2πdfTStdwhich amounts to a simple phase rotation that depends only on the frequency shift.
A fixed or adjustable threshold is put at the output of the correlator. If this threshold is crossed, the receiver declares that it has “acquired” the reference and not the time at which the threshold has been crossed, from which it will deduce the time reset to be made.
FIG. 5 gives a schematic view of an exemplary embodiment according to the prior art.
The receiver tries to recombine the output of the correlator gathered throughout (or almost throughout) the duration of the watch on the frequencies F(1), F(2), . . . , F(M). It must be noted that these are coarse outputs of the correlator, and that the detection threshold mentioned further above is not used.
Given that, on each of these frequencies and if the time shift of the receiver is not too high, the reference stage must reach the instants T(1), T(2), . . . , T(M) plus or minus an unknown constant delay, the receiver then proceeds after the watch on the last frequency (M) as follows:                it carries out a point-by-point summation of the output of the M memories, each delayed by the appropriate quantity (the delay lines are not “physically” indispensable: they may be replaced by appropriate shifts in the memorized read addresses),        it compares all the sums obtained with a fixed or variable threshold to detect the presence of the signal, as well as its temporal shift.This embodiment is very costly in terms of memory, especially if the product of the sampling rate (proportional to the modulation speed, the duration of watch (proportional to the temporal uncertainty) and the number of frequencies to be monitored (especially great as the conditions of reception are more severe) is high (amounting to several tens or hundreds of thousands).Furthermore, for high Doppler shifts, it becomes necessary, even in adopting a differential demodulation pattern at output as referred to further above, to use several devices identical to this one, each corresponding to a given range of frequency shifts (“the Doppler window”), all the devices covering the entire range of Doppler shifts that the system must cope with.        
FIG. 6 shows a variant of the system described in FIG. 5, necessitating M times less memory, in which a processing operation is carried out continuously on each frequency to be watched:                on the first frequency to be watched, the operation is limited to the direct storage of the signal coming from the correlator,        on the following frequencies, the output of the correlator delayed by the appropriate quantity is added to the already stored correlations;        at the last frequency, the output of the accumulator is compared with a presence detection threshold.Since some of the differences in delays T(m)−T(1) (with m=2 . . . M) may be negative, and if the reference stages can be located anywhere in the basic pattern, the memory for storing the correlations on the frequency F1 must be widened to contain all the samples corresponding to a basic pattern, or else, symmetrically, if the size of this memory is not modified, the range of temporal uncertainty in which it is possible to unmistakably detect the M reference stages is reduced by the duration of a basic pattern.        
However, this embodiment does not avert the need, as the case may be, to cover several Doppler windows and hence make several copies of the system, each being assigned to a Doppler window.
The system and method according to the invention rely on a novel approach to the processing of signals.
They can be applied especially to systems in which the conditions of reception are especially unfavorable (with low signal-to-noise ratios and a very high Doppler shift) where the acquisition by detection of a single reference stage is improbable and where it is therefore necessary to simultaneously detect several stages to obtain the required level of performance.